1. Technical Area
The present invention relates to coupling of pump light into an optical amplifier or a laser and, in particular, to coupling from a multimode laser diode in order to optically pump an optical amplifier or laser.
2. Discussion of Related Art
Use of directed energy beams, such as those produced by amplifiers or lasers, are diverse and include applications in a wide range of fields, including biotechnology, medicine, semiconductor processing, manufacturing, image recording and defense. In biotechnology, directed energy beams are used, for example, in flow cytometry, DNA sequencing, confocal microscopy, and hematology. Medical applications include use in ophthalmology, non-invasive surgery, and photodynamic therapy. In the semiconductor industry, applications include wafer inspection, rapid thermal processing, and scribing or marking. Image recording applications include, for example, high-speed printing, photo-processing, film subtitling and holography. Industrial applications include, for example, rapid prototyping, materials processing and scribing or marking. Additionally, military applications include range finding, target designation, lidar, and chemical or biological threat detection. The graphics and printing industry, one of the largest businesses in the world, has a need for inexpensive laser systems for use in applications such as thermal graphics. Such applications require a highly reliable, low noise laser or optical amplifier at a low cost.
Typically, applications for directed energy beams require a laser or optical amplifier. An optical amplifier differs from a laser by the absence of a laser cavity. Both devices typically require an active optical material, for example rare-earth doped YAG, ruby (Al2O3:Cr), or other material, which can be optically “pumped,” such that energy can be stored in the excited states of the active atoms or molecules by an optical pump source. Amplification of input optical radiation or stimulated emission for lasing then occurs when the same optical energy stored in the excited states is coupled to the incident optical beam.
FIG. 1A shows an example of a side-pumped laser 100. Laser material 101 is positioned in a laser cavity defined by mirrors 102 and 103 and is pumped by diode array 104. Diode array 104 includes a series of laser diodes 105-1 through 105-N positioned to illuminate all or most of laser material 101. There are a variety of choices for laser diodes and laser diode arrays available to pump Nd or Yb doped YAG, for example. In most applications, Nd:YAG is pumped at about 808 nm and Yb:YAG is pumped at about 940 nm. Choices for diode array 104 include 10-40W arrays, 40-50W single bars, and 240-600W stacked bars, for example. Arrays can also be formed from readily available 1-2W single laser diodes.
FIG. 1B illustrates the optical density in a cross section of laser material 101 in side-pumped laser 100 of FIG. 1A. As is shown in FIG. 1B, the optical density is greatest in the center of laser material 101 where the laser beam is located. However, much of the pump energy is dissipated in areas of laser material 101 that are not actively involved in the lasing process. Therefore, side pumping techniques are inherently inefficient.
As is illustrated in FIG. 1A, the laser beam is directed between mirrors 102 and 103, where a percentage of the beam is transmitted through mirror 103. FIG. 2 illustrates the shape of a laser beam in a laser cavity such as in laser 100. The closer the laser beam is to its diffraction limit in laser material 101, the greater the depth of field and the smaller the diameter of beam handling optics (for example mirrors 102 and 103) required to transmit the beam. The ratio of the divergence of the laser beam to that of a theoretically diffraction limited beam of the same waist size in the TEM00 mode is usually given as M2=(Θ/θ), where Θ is the divergence angle of the laser beam and θ is the divergence angle of the theoretical laser beam. The angular size of the laser beam in the far field will be M2 times the size calculated for a perfect Gaussian beam, i.e. Θ=M2(2λ/W0) for a beam waist diameter of 2W0.
FIG. 1C illustrates an end-pumping arrangement for pumping laser material 101. In the arrangement shown in FIG. 1C, laser material 101 is again placed in a laser cavity formed by mirrors 102 and 103. The laser optical energy transmitted through mirror 103 is reflected by a dichroic beam splitter 114 to form the beam. Optical energy from pump source 116 is incident on lens 115 and passes through dichroic beam splitter 114 and mirror 103 to Locus in a nearly diffraction limited region of laser material 101. The beam from pump source 116 is reduced to a size and shape that resembles the shape of the laser beam shown in FIG. 2 in active material 101. Additionally, a second pump source 110 can be focused by lens 113 through mirror 102 and into laser material 101. In some embodiments, additional optical energy can be coupled into laser material 101 from pump source 111 using a polarizing beam splitting cube 112, which transmits light from pump source 110 while reflecting light from pump source 111.
A cross section of laser material 101 illustrating optical power concentration is shown in FIG. 1D. As can be seen in FIG. 1D, nearly all of the pump power, as well as the laser beam, is focused in the active region of laser material 101, where the laser beam produced by laser 117 is produced.
As is pointed out in U.S. Pat. No. 4,710,940 to D. L. Sipes, Jr, issued on Dec. 1, 1987, to a first approximation, and not being limited by theory, the higher the pump power density the more efficient is the use of pump power. This concept is illustrated in the graphs shown in FIGS. 1E and 1F. FIG. 1E shows the photon conversion efficiency (i.e., the number of pump photons versus the number of output laser light photons) with increasing mirror reflectivity at various input optical power densities. Higher mirror reflectivity increases the optical power density within the laser cavity. At higher pump power densities, higher efficiencies result. FIG. 1F shows photon conversion efficiencies as a function of pump power for various spot sizes, which shows the same trend of higher efficiency with optical density as does the graph shown in FIG. 1E. Spot size refers to the diameter of the optical pump in the optically active laser material.
Table I shows typical power usage and lifetime characteristics for a side pumped laser 100 as shown in FIG. 1A, an end-pumped laser 117 as is shown in FIG. 1C, and a lamp pumped laser. As expected, the diode end-pumped laser 117 has the greater efficiency. However, end-pumped laser systems have more optical components and therefore are difficult to align.
Typically, the optical beam from a laser diode outputs is highly assymmetric. Therefore, light from the diodes is difficult to couple into the active material, e.g. laser material 101, of an optical amplifier or a laser. However, as shown in Table I, the lifetimes, efficiency, and expense of various laser diode configurations make them very attractive as pump sources for optically active devices.
TABLE ILampDiode End-Diode Side-PumpedPumpedPumpedPower to Pump5000W2.5W50WSourcePower to Cooling500W2.5W50WSystemPower from Pump3500W1.25W20WSourceSingle-Mode10W0.8W10WPower from LaserWall-Plug Effi-0.2%16%10%ciencyCoolingWaterFree AirForced AirPower Consumed/500kW-hr6.5kW-hr10kW-hrOutput kW-hrCost of Light$200$400$1000SourceLifetime of Light200hrs20,000hrs10,000hrsSource
Multimode laser diodes are highly desirable optical pump sources as they are inexpensive to manufacture and are capable of producing much higher power levels than single mode lasers. Multimode lasers are more reliable than single-mode lasers as they have lower output power densities reducing the risk of catastrophic facet damage, the primary cause of laser diode failure. However the light emitted by a multimode laser diode is very asymmetric. Typically, the laser diode emitting aperture has dimensions on the order of 1 μm×100 μm. It is very difficult and costly to collect and couple light emitted by a multimode laser diode into the end facet of a single-mode optical waveguide or fiber.
Most conventional waveguide amplifiers and lasers include one or more waveguide cores doped with active elements, such as Er, Yb, Nd and Tm, and are designed such that the waveguide can support coaxially propagating single-mode output and pump light. The output power of a single-mode, single laser pumped amplifier or laser is often limited to about 20 dBm (100 mW) by the power levels of available single-mode pump lasers. Single-mode pump lasers require more precision manufacturing tolerances and are consequently more expensive to produce than multimode lasers. As a result complex and costly schemes are required to pump arrays of waveguide optical amplifiers and lasers. Pump light has to be distributed to each amplifier channel or laser element, requiring combinations of splitters, combiners, taps, monitors and associated control electronics to effectively manage the distribution. Polarization sensitivity of waveguide elements further complicates the distribution process.
Therefore, there is a need for optical laser devices capable of efficiently coupling light from a laser diode into the active region of a laser cavity that is cost effective and reliable, and that produces high optical output power.